1. Field of the Invention
The present invention relates to a heat-sensitive recording material, and particularly, to a heat-sensitive recording material having high heat sensitivity and being superior in high-speed recording.
2. Description of the Prior Art
Heat-sensitive recording materials which make use of the heat color-development reaction of a colorless to pale leuco dye with an organic acid are disclosed in JP-B-43-4160, JP-B-45-14039, etc. and put to wide and practical use.
In these recording fields, recently, demands for an increase in the rate and density of recording have become so strong that the development of color-development recording apparatus which can meet these demands and recording materials suitable therefor is also being strongly demanded. In these apparatus, reducing the heat amount supplied to the thermal head is planned for energy saving and an improvement in the durability of the thermal head. Because of this, for the heat-sensitive recording materials used is also being required a sufficient sensitivity enough to give clear color-developed records (images) with smaller amount of heat.
Practically, however, when the heat-sensitive recording paper is used in thermal printers, insufficient color-developing sensitivity and slowed-down printing speed are observed in many cases. In order to improve these drawbacks, additives are in use. For example, examples of use of various additives are disclosed in JP-B-50-14531, JP-A-60-82382, JP-A-59-73990, JP-A-60-56588, JP-A-58-98285, JP-A-60-176794, JP-A-59-184692, JP-A-58-87094, JP-A-56-72996, JP-A-60-178086, etc. By the use of these compounds, there is observed a tendency for the developed color depth to become strong for the heat amount supplied. However, there are observed the following drawbacks: The fastnesses to heat, moisture, water, plasticizer, etc. of the color-developed parts are inferior; these compounds crystallize out as white crystals on the recorded (color-developed) images during storage after heat- sensitive color-development (the so-called blooming); and color-development easily takes place in a low-temperature region to form fog on the ground part, which lowers the contrast between the color-developed image and the ground part on the heat-sensitive paper. As described above, when additives are added in order to improve the color-developing sensitivity, the foregoing problems arise in many cases in contrary to the object. For this reason, a satisfactory method has not yet been established although various additives have been proposed.